Light radiation protection material for a large energy application field

ABSTRACT

The invention relates to a lead substitute material for radiation protection purposes, wherein the lead substitute material comprises from 12 to 22 wt. % matrix material, from 0 to 75 wt. % Sn or Sn compounds, from 0 to 73 wt. % W or W compounds, from 0 to 80 wt. % Bi or Bi compounds, and wherein not more than one of the constituents is 0 wt. %, for nominal overall lead equivalents of from 0.25 to 2.00 mm. The invention relates further to a lead substitute material that additionally comprises one or more of the elements Er, Ho, Dy, Tb, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I, Ta, Hf, Lu, Yb, Tm, Th, U and/or their compounds and/or CsI.

The invention relates to a lead substitute material for radiationprotection purposes in the energy range of an X-ray tube having avoltage of from 60 to 140 kV.

Conventional radiation protection clothing for use in X-ray diagnosticsmostly contains lead or lead oxide as the protective material.

It is desirable to replace this protective material by other materialsfor the following reasons in particular:

On the one hand, because of its toxicity, lead and the processingthereof result in considerable damage to the environment; on the otherhand, because of lead's very great weight, protective clothing isnecessarily very heavy, resulting in considerable physical strain on theuser. When wearing protective clothing, for example during medicaloperations, the weight is of great importance in terms of wear comfortand the physical strain on the doctor and his assistants.

For that reason, a substitute material for lead in radiation protectionhas been sought for many years. The use of chemical elements orcompounds thereof having an atomic number from 50 to 76 is predominantlyproposed.

DE 199 55 192 A1 describes a process for the production of a radiationprotection material from a polymer as matrix material and the powder ofa metal having a high atomic number.

DE 201 00 267 U1 describes a highly resilient, lightweight, flexible,rubber-like radiation protection material, wherein chemical elements andoxides thereof having an atomic number greater than or equal to 50 areadded to a specific polymer.

In order to reduce the weight as compared with conventional lead aprons,EP 0 371 699 A1 proposes a material that likewise contains, in additionto a polymer as the matrix, elements having a higher atomic number. Alarge number of metals is mentioned therein.

DE 102 34 159.1 describes a lead substitute material for radiationprotection purposes in the energy range of an X-ray tube having avoltage of from 60 to 125 kV.

Depending on the elements used, the degree of attenuation or the leadequivalent (International Standard IEC 61331-1, Protective devicesagainst diagnostic medical X-radiation) of the material in question insome cases exhibits a pronounced dependency on the radiation energy,which is a function of the voltage of the X-ray tube.

Compared with lead, the absorption behaviour of lead-free materials insome cases differs considerably depending on the X-ray energy. For thisreason, an advantageous combination of different elements is required inorder to imitate the absorption behaviour of lead while at the same timemaximising the saving in terms of weight.

Accordingly, as compared with lead, known radiation protection clothingof lead-free material possesses a more or less pronounced fall inabsorption below 70 kV and above 110 kV, in particular above 125 kV.This means that, in order to achieve the same shielding effect as withlead-containing material, the protective clothing is required to have ahigher weight per unit area for this range of the tube voltage.

For this reason, the field of application of commercial lead-freeradiation protection clothing is generally limited.

In order to be able to replace lead for radiation protection purposes,an absorption behaviour is required, in relation to lead, that is asuniform as possible over a relatively large energy range, becauseradiation protection materials are conventionally classified accordingto the lead equivalent, and radiation protection calculations arefrequently based on lead equivalents.

In the case of a lead substitute material composed of protective layers,the overall lead equivalent is understood as being the lead equivalentof the sum of all the protective layers. The overall nominal leadequivalent is understood as being the lead equivalent to be indicated bythe manufacturer according to DIN EN 61331-3 for personal protectiveequipment.

Matrix material is understood as being the carrier layer for theprotective materials, which layer may consist, for example, of rubber,latex, flexible or rigid polymers.

In particular X-ray applications, such as computed tomography and in thecase of bone density measurements, but also in baggage checking devices,X-ray voltages up to 140 kV occur.

The object of the present invention is to replace lead as a radiationprotection material in respect of its shielding properties over a broadenergy range of an X-ray tube, that is to say over a large energy range,and at the same time to achieve as great a reduction in weight aspossible. Only materials that are environmentally friendly compared withlead are to be used.

The object of the invention is achieved by a lead substitute materialfor radiation protection purposes in the energy range of an X-ray tubehaving a voltage of from 60 to 140 kV, wherein the lead substitutematerial comprises from 12 to 22 wt. % matrix material, from 0 to 75 wt.% tin or tin compounds, from 0 to 73 wt. % tungsten or tungstencompounds, from 0 to 80 wt. % bismuth or bismuth compounds and whereinnot more than one of the constituents is 0 wt. %. The mixture coversnominal overall lead equivalents of from 0.25 to 2.0 mm.

In order to achieve the object it was therefore necessary to find achoice of material and a choice of the amount thereof that is ableeffectively to shield the X-radiation even in the high energy range.

It has been found, surprisingly, that the absorption effect in the caseof high energies is substantially improved by high proportions oftungsten and/or bismuth in the lead substitute material.

In a preferred embodiment of the invention, the lead substitute materialis characterised in that it comprises from 12 to 22 wt. % matrixmaterial, from 0 to 39 wt. % Sn or Sn compounds, from 0 to 60 wt. % W orW compounds and from 0 to 60 wt. % Bi or Bi compounds, wherein not morethan one of the constituents is 0 wt. %.

In a particularly preferred embodiment of the invention, the leadsubstitute material is characterised in that it comprises from 12 to 22wt. % matrix material, from 0 to 39 wt. % Sn or Sn compounds, from 16 to60 wt. % W or W compounds and from 16 to 60 wt. % Bi or Bi compounds.

In a further preferred embodiment of the invention, the lead substitutematerial is characterised in that it comprises from 12 to 22 wt. %matrix material, from 40 to 60 wt. % Sn or Sn compounds, from 7 to 15wt. % W or W compounds and from 7 to 15 wt. % Bi or Bi compounds.

In a further particularly preferred embodiment of the invention, thelead substitute material is characterised in that it additionallycomprises up to 40 wt. % of one or more of the following elements: Er,Ho, Dy, Tb, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I and/or their compoundsand/or CsI.

Table 1 below shows the mass attenuation coefficients of lead-freeprotective materials outside the absorption edges at different photonenergies. The elements that are advantageously to be used in the case ofa particular energy are underlined. Energy (keV) Sn Gd Er W Bi 40 19.42 6.92  8.31 10.67 14.95 50 10.70  3.86  4.63  5.94  8.38 60  6.56 11.7513.62  3.71  5.23 80  3.03  5.57  6.48  7.81  2.52 100  1.67  3.11  3.63 4.43  5.74 150  0.61  1.10  1.28  1.58  2.08

By means of the lead substitute material, which additionally comprisesone or more elements Er, Ho, Dy, Tb, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, Iand/or their compounds and/or CsI, a particularly pronounced increase inthe absorption effect is achieved. In this manner, the weight of theprotective clothing can be substantially reduced.

In order to achieve the described properties it is possible according toTable 1 to combine the individual elements in such a manner that aparticular energy range is covered or that as uniform a progression aspossible of the attenuation is obtained over a relatively large energyrange.

Surprisingly, it has been found that, when the above-mentionedadditional elements or their compounds are used in the lead substitutematerial, a superproportional increase in the protective effect occurs,especially when their amount by weight in the lead substitute materialis from 20% to 40%.

In a further preferred embodiment of the invention, the lead substitutematerial is characterised in that it additionally comprises up to 40 wt.% of one or more of the following elements: Ta, Hf, Lu, Yb, Tm, Th, Uand/or their compounds.

In the case of the metals Er, Ho, Dy, Tb, Gd, Eu, Sm, La, Ce, Nd, Ba, I,Ta, Hf, Lu, Yb, Tm, Th, U that can additionally be used in the leadsubstitute material, it is also possible to use metals and/or theircompounds and/or CsI having a relatively low degree of purity, as areobtained as waste products.

In DIN EN 61331-3 a deviation downwards from the nominal lead equivalentis not permitted. Only the German version of the standard permits anexception, namely a deviation of 10% from the nominal lead equivalent.For these reasons it is desirable for the progression of the leadequivalent in the case of a lead substitute material to be as flat aspossible over the energy.

A fall in the lead equivalent below the nominal lead equivalent or belowthe lower tolerance limit means that the radiation protection materialcannot be used at the tube voltages in question because the shieldingeffect is too low. In such a case it is necessary, as an alternative, toincrease the weight per unit area of the lead substitute material sothat the permitted tolerances of DIN EN 61331-3 are met. However, anincrease in the weight per unit area is regarded as disadvantageous.

A further possibility consists in limiting the field of application inrespect of the energy or the tube voltage.

A further object of the present invention was, therefore, to selectelements or their compounds in such a manner that as small a fall aspossible occurs in the lead equivalent in the desired energy use range,taking into account the availability of the particular elements inquestion or their compounds.

The relative effectiveness N_(rel) as an increase in the lead equivalent(LE), based on a standardised mass loading of 0.1 kg/m², was determinedin a series of tests on a number of materials and compiled in Table 2below. It indicates the attenuation properties of the individualelements more clearly than the above-described mass attenuationcoefficients, because here the absorption also flows in in the immediateregion of the particular absorption edges. TABLE 2 Rise in LE N_(rel)from 60 to Mean LE increase based on 80 kV 0.1 kg/m² (rel. Pb) based60-90 60-125 100-125 125-150 on 0.1 Material kV kV kV kV kg/m² Group Sn1.64 1.30 0.96 0.80 −0.005 A Bi 1.41 1.27 1.13 1.17 −0.005 A W 0.91 1.071.25 1.07 +−0.000 A Gd 1.85 2.05 2.27 1.56 +0.007 B Er 1.20 1.45 1.701.36 +0.009 B

Surprisingly, it is thereby shown that the elements or their compoundscan be classified as follows:

-   -   Group A: materials having relatively low effectiveness with        values of N_(rel)<1.2-1.6 mm LE per 0.1 kg/m² and a low or        negative increase from 60 to 80 kV. These elements or their        compounds include Sn, Bi and W.    -   Group B: materials having relatively high effectiveness with        N_(rel)≧1.3 mm LE per 0.1 kg/m² and a large increase from 60 to        80 kV.

In a particularly preferred embodiment of the invention, the energyrange from 60 to 140 kV, which corresponds to the most frequentapplications of X-radiation, is therefore divided into several ranges,some of which overlap:

1. 60-90 kV Energy Range

In this energy range there take place predominantly dental applicationsof the single exposure technique and panoramic layer technique.

2. 60-125 kV Energy Range

The most frequent X-ray investigations and X-ray interventions liewithin this energy range, such as angiography, computed tomography,cardiac catheter investigations, inverventional radiology, thorax hardradiation technique.

3. 100-125 kV Energy Range

Most computed tomographs fall within this energy range.

4. 125-150 kV Energy Range

This is an energy range for special applications, such as specialcomputed tomographs, bone density measurements, special thorax hardradiation technique and nuclear medical diagnostics.

Lead-free protective clothing that can be used in only a particularenergy range is to be correspondingly labelled by the manufacturer.

In an embodiment of the lead substitute material for radiationprotection purposes in the energy range of an X-ray tube having avoltage of from 60 to 90 kV, the lead substitute material for nominaloverall lead equivalents of from 0.25 to 0.6 mm is characterised in thatit comprises from 12 to 22 wt. % matrix material, from 49 to 65 wt. % Snor Sn compounds, from 0 to 20 wt. % W or W compounds, from 0 to 20 wt. %Bi or Bi compounds and from 2 to 35 wt. % of one or more of the elementsGd, Eu, Sm, La, Ce, Nd, Cs, Ba, I, Pr and/or their compounds and/or CsI.The energy range is preferably that of an X-ray tube of a dental X-raydevice.

In a particular embodiment of the present invention, the lead substitutematerial comprises from 2 to 25 wt. % I, Cs, Ba, La, Ce, Pr and/or Ndand/or their compounds and/or CsI.

In the case of the relatively narrow energy range, Table 2 has shownthat Sn is the most effective of the group A elements. From group B,preference is given to Gd, although CsI also yielded a lead substitutematerial having very good properties.

60-125 kV Energy Range (General X-Ray Range):

From Table 2 it is advantageously possible, for example, to selectelements with a small and a large increase in the lead equivalent, sothat the progressions of the lead equivalent remain as flat as possibleover the entire range. A certain excessive increase at 80 and 100 kVcannot be avoided physically.

It is therefore possible to combine one or more elements or theircompounds of group A with one or more elements or their compounds ofgroup B in an optimum manner, the choice being made according to theefficiency of the shielding, according to the availability of theelement in question or its compound and according to as constant aspossible a progression of the lead equivalent.

The proportion of the A elements or their compounds is dependent on theproportion of the B elements or their compounds. Accordingly, when theproportion of a B element is increased, the relative proportion byweight of an A element having the opposite energy behaviour must also bemarkedly increased in order to keep the progression of the leadequivalent as flat as possible over the energy.

For example, with a proportion of over 20 wt. % of B elements or theircompounds, the proportion of Sn or Bi should rise to over 40 wt. % inorder to ensure low energy dependency.

100-140 kV Energy Range:

This is the energy range for most newer computed tomographs.

In a particularly preferred embodiment of the invention, the leadsubstitute material for radiation protection purposes in the energyrange of an X-ray tube having a voltage of from 100 to 140 kV ischaracterised in that the lead substitute material for nominal overalllead equivalents of from 0.25 to 0.6 mm comprises from 12 to 22 wt. %matrix material, from 40 to 73 wt. % Bi and/or W or their compounds andfrom 5 to 38 wt. % of one or more of the following elements: Gd, Eu, Er,Hf and/or their compounds.

High protective effects, or low weights per unit area, can be achievedby the use of the elements or their compounds that develop theirgreatest shielding effect specifically within this small energy range.For reasons of availability, a larger proportion of the elements ortheir compounds of group A is to be combined with a smaller proportionof the elements or their compounds of group B, a flat energy progressionof the lead equivalent being less important in this case because of therelatively small energy window.

125-150 kV Energy Range:

This range relates to special applications in radiology and nuclearmedicine. The weight per unit area of the radiation protection apron isnot at the forefront of the optimisation in this case because theprotective clothing is generally worn for only a short time here orfixed radiation protection screens are used.

The selection of the elements or their compounds is carried outaccording to the above-mentioned criteria. Gd and Er in combination withBi yield very good results. The effect of W in this range is too low.

In summary, it can therefore be stated that the composition ofprotective materials for individual energy ranges corresponding to themost frequently occurring X-ray applications can advantageously beoptimised by division.

In a further preferred embodiment of the invention, the lead substitutematerial comprises a structure of at least two protective layers ofdifferent compositions which are separate or joined together, wherein atleast in one layer at least 50% of the total weight consists of only oneelement from the group Sn, W and Bi or their compounds.

In particular, the lead substitute material comprises a structure of atleast two protective layers of different compositions which are separateor joined together, wherein at least in one layer at least 50% of thetotal weight consists only of at least 40 wt. % Sn or its compounds andat least 10 wt. % I, Cs, Ba, La, Ce, Pr and/or Nd and/or their compoundsand/or CsI. A layer that comprises from 40 to 50 wt. % Sn and from 10 to20 wt. % cerium is particularly advantageous.

In a further preferred embodiment of the invention, the lead substitutematerial is characterised in that it comprises a structure of at leasttwo protective layers of different compositions which are separate orjoined together, wherein the layer(s) more remote from the bodycomprise(s) predominantly the elements or their compounds having ahigher X-ray fluorescent yield and the protective layer(s) close to thebody comprise(s) the elements or their compounds having a lower X-rayfluorescent yield.

In the irradiation of materials with X-radiation, characteristicX-radiation is excited as fluorescence radiation. The fluorescent yieldis dependent on the atomic number. This fluorescence content results inadditional exposure of the skin and the organs lying immediately beneathit to radiation. From measurements on protective clothing it has beenfound that elements having lower atomic numbers in particular, in thepresent case therefore in particular Sn, fluoresce particularlystrongly. In the case of a layered structure of the radiation protectionmaterial it is advantageously possible to carry out layering accordingto elements, so that the elements having the lowest fluorescent yieldlie on the skin side.

The fluorescence content, also referred to as the build-up factor, ofcommercial lead-free protective materials (material B) is shown in Table3 below in comparison with a material composed according to theprinciple described herein (material A). As will be seen, the build-upfactor can reach values up to 1.42. That is to say, the exposure of theskin is in this case increased by 42%, owing to the fluorescencecontent. TABLE 3 kV Material A Material B 80 1.15 1.42 90 1.14 1.35 1001.14 1.32 110 1.16 1.36

In a further particularly preferred embodiment of the invention, thelead substitute material is characterised in that it comprises astructure of protective layers of different compositions.

The lead substitute material can comprise a structure of at least twoprotective layers of different compositions which are separate or joinedtogether, wherein the protective layer(s) more remote from the bodycomprise(s) predominantly the elements having a lower atomic number, ortheir compounds, and the protective layer(s) close to the bodycomprise(s) predominantly the elements having a higher atomic number, ortheir compounds.

The lead substitute material may also comprise a structure of at leastthree protective layers of different compositions which are separate orjoined together, wherein the protective layer(s) more remote from thebody and the protective layer(s) close to the body comprisepredominantly the elements having higher atomic numbers, or theircompounds, and there is arranged in the middle at least one protectivelayer comprising predominantly elements having lower atomic numbers.

A barrier layer of a material having higher atomic numbers, such as, forexample, bismuth or tungsten, is accordingly located, for example, onthe outside on both sides. Between those layers there is(are) (a)layer(s) of a material having a lower atomic number. The fluorescentradiation that forms therein is therefore effectively shielded on bothsides and cannot pass to the outside.

Alternatively, it is also possible to provide a layer structure thatcomprises at least one highly concentric, compressed powder layercomprising a mixture of the above-mentioned protective materials, and atleast two carrier layers located on both sides of the powder layer. Thepowder layer contains as small an amount of matrix material as possible.The carrier layers may be composed of matrix material. Examples ofsuitable materials are polymers, such as latex or elastomers. Thecarrier layers increase the mechanical stability, while the concentratedfilling improves the radiation-shielding effect. FIG. 4 shows thislayered structure with a highly compressed protective-material layer 2as the core and the external carrier layers 1.

The lead substitute material may also be characterised in that a weaklyradioactive layer is embedded between two non-radioactive protectivelayers which are separate from or joined to the radioactive layer.

It is possible to use as the elements or their compounds of group B forshielding high-energy radiation also the actinoids thorium or uranium,the latter, for example, in the form of depleted uranium. They have ahigh shielding effect in the 125-150 kV energy range but are themselvesweakly radioactive.

The effect of the intrinsic radiation can be moderated by embedding theradioactive layer between two non-active layers of Bi. The amount ofinherent exposure to thorium or uranium should in most cases be low andhence negligible. It is here necessary to weigh up the advantagesachieved by the elimination of lead and by the higher protective effectagainst the low inherent exposure.

In a further preferred embodiment of the invention, the lead substitutematerial is characterised in that the metals or metal compounds aregranular and their particle sizes exhibit a 50th percentile according tothe following formula $D_{50} = {\frac{d \cdot p}{10}{mm}}$wherein D₅₀ represents the 50th percentile of the particle sizedistribution, d represents the layer thickness in mm and p representsthe proportion by weight of the particular material component in thetotal weight, and the 90th percentile of the particle size distributionD₉₀≦2·D₅₀.

When the lead equivalents of protective layers consisting of metalpowders or powders of metal compounds were measured it was found,surprisingly, that the transmittance of the layer consisting of granularsubstances is higher compared with a film layer with the same massloading. This mainly concerns the lower energy range of 60-80 kV. Athigher energies, the local differences in transmittance, that is to saythe X-ray contrast, become increasingly smaller.

For example, with an Sn content of 30%=0.3 and a layer thickness of 0.4mmD ₅₀=0.4 mm·0.3=0.012 mm=12 μm.

Moreover, the 90th percentile of the particle size distribution shouldnot be greater than 2·D₅₀=24 μm.

Materials having a low proportion by weight must therefore also possessa small particle size, that is to say must be very finely divided, inorder to develop an optimum protective effect.

If this effect is utilised, the weight of a radiation protection aproncan be reduced still further.

The material according to the invention can advantageously be used, forexample, in protective gloves, patient coverings, gonad protection,ovary protection, protective dental shields, fixed lower-bodyprotection, table attachments, fixed or movable radiation protectionwalls or radiation protection curtains.

The invention is to be explained in greater detail hereinbelow by meansof examples.

EXAMPLE 1

FIG. 1 shows the lead substitute material according to the inventioncomprising 22 wt. % tin, 27 wt. % tungsten, 4 wt. % erbium and 15 wt. %matrix material. This lead substitute material is denoted 2 in FIG. 1. 1denotes a commercial material composed of 65 wt. % antimony, 20 wt. %tungsten and 15 wt. % matrix material.

FIG. 1 shows a weight comparison of lead substitute materials at anominal lead equivalent of 0.5 mm.

It will be seen from FIG. 1 that the weight per unit area required toachieve a nominal lead equivalent of 0.5 mm increases by only about 7%between 100 and 140 kV in the case of the material according to theinvention, whereas the increase is considerably greater in the case ofthe comparison material.

EXAMPLE 2

FIG. 2 shows the lead substitute material according to the inventioncomprising 20 wt. % tin, 36 wt. % tungsten, 29 wt. % bismuth and 15 wt.% matrix material. This lead substitute material is denoted 2 in FIG. 2.1 denotes a commercial material composed of 70 wt. % tin, 10 wt. %barium and 20 wt. % matrix material.

FIG. 2 shows a weight comparison of lead substitute materials at anominal lead equivalent of 0.5 mm.

It will be seen from FIG. 2 that the weight per unit area required toachieve a nominal lead equivalent of 0.5 mm increases only by about 9%between 100 and 140 kV in the case of a material according to theinvention, whereas the increase is about 60% in the case of thecomparison material.

EXAMPLE 3

Lead-free, lightweight radiation protection apron for the dental rangeof 60-90 kV Pb nominal lead equivalent 0.5 mm.

A lead-free radiation protection apron comprising 59 wt. % Sn, 24 wt. %Gd, 1 wt. % W and 16 wt. % matrix material was produced.

The radiation protection effect corresponded to that of a correspondinglead apron at a weight per unit area of only 4.4 kg/m², a reduction ofabout 35%.

EXAMPLE 4

Lead-free, lightweight radiation protection apron for the 60-125 kVapplication range.

A radiation protection apron comprising 50 wt. % Sn, 11 wt. % W, 23 wt.% Gd and 16 wt. % matrix material was produced.

For a nominal lead equivalent of 0.5 mm lead, a weight per unit area of4.5 kg/m² was obtained; for a nominal lead equivalent of 0.35 mm lead, aweight per unit area of 3.3 kg/M² was obtained; and for a nominal leadequivalent of 0.25 mm lead, a weight per unit area of 2.4 kg/m² wasobtained.

EXAMPLE 5

Lead-free, lightweight radiation protection apron for the 60-125 kVapplication range.

A radiation protection apron comprising 40 wt. % Bi, 20 wt. % Sn, 24 wt.% Gd and 16 wt. % matrix material was produced.

For a nominal lead equivalent of 0.5 mm lead, a weight per unit area of5.0 kg/m² was obtained.

Commercial lead-free radiation protection aprons exhibit weights perunit area of from 5.4 to 6.1 kg/m² at nominal lead equivalents of 0.50mm. Conventional lead/rubber material has a weight per unit area of 6.75kg/m².

The fundamental advantage of the present invention thus becomes clear,according to which the protective clothing can be made considerablylighter. This is a very important advantage in particular when using theprotective clothing for a period of several hours.

In addition, if the user is working at tube voltages of 80-100 kV, thelead equivalent is about 20% above the nominal value of 0.5 mm Pb of acorresponding lead apron. This means an additional increase in theradiation protection.

EXAMPLE 6

Lead-free, lightweight radiation protection apron for computedtomography.

A radiation protection apron comprising 40 wt. % Bi, 10 wt. % W, 34 wt.% Gd and 16 wt. % matrix material was produced.

A surprisingly low weight per unit area of only 4.6 kg/m² was obtainedfor a nominal lead equivalent of 0.5 mm.

EXAMPLE 7

Lead-free, lightweight apron for nuclear medical applications.

A nuclear medical apron was produced from 50 wt. % Bi, 25 wt. % Gd, 9wt. % Er and 16 wt. % matrix material.

The weight per unit area was 4.8 kg/M² for a nominal overall leadequivalent of 0.5 mm.

EXAMPLE 8

FIG. 3 shows the calculated relative weights per unit area of theprotective clothing according to the invention with nominal leadequivalents of 0.5 mm according to Examples 3, 4 and 6, in comparisonwith a lead apron with a 0.5 mm lead equivalent. It will be seen fromthe figure that the protective aprons for dental use, general X-ray andcomputed tomography (CT) each exhibit the lowest weight per unit area inthe intended energy range.

In addition, if the user is working at tube voltages of 80-100 kV, thelead equivalent is about 20% above the nominal value of 0.5 mm Pb of acorresponding lead apron. This means an additional increase in theradiation protection.

EXAMPLE 9

Lead-free, lightweight apron in the energy range of from 60 to 120 kVhaving a two-layer structure.

The matrix content is 15 wt. %.

The following composition was chosen for the protective material layers:Material weight Layer Element/compound (kg/m²) Fluorescent layer Sn 1.20(outside) Gd (oxide) 0.72 cerium (oxide) 0.48 Barrier layer Bi 1.44(inside) W 0.48 Gd (oxide) 0.48

A low weight per unit area of only 4.8 kg/m² was obtained for a leadequivalent of 0.5 mm.

EXAMPLE 10

Lead-free, lightweight apron in the energy range of from 60 to 120 kVhaving a two-layer structure.

The matrix content is 15 wt. %.

Following composition: Material weight Layer Element/compound (kg/m²)Fluorescent layer Sn 1.20 (outside) Gd (oxide) 0.48 cerium (oxide) 0.72Barrier layer Bi 1.44 (inside) Gd (oxide) 0.96

A low weight per unit area of only 4.8 kg/m² was obtained for a leadequivalent of 0.5 mm.

1. Lead substitute material for radiation protection purposes in theenergy range of an X-ray tube having a voltage of from 60 to 140 kV,wherein for nominal overall lead equivalents of from 0.25 to 2.0 mm thelead substitute material comprises from 12 to 22 wt. % matrix material,from 0 to 75 wt. % Sn or Sn compounds, from 0 to 73 wt. % W or Wcompounds, from 0 to 80 wt. % Bi or Bi compounds, and wherein not morethan one of the constituents is 0 wt. %.
 2. Lead substitute materialaccording to claim 1, characterised in that the lead substitute materialcomprises from 12 to 22 wt. % matrix material, from 0 to 39 wt. % Sn orSn compounds, from 0 to 60 wt. % W or W compounds, from 0 to 60 wt. % Bior Bi compounds, and wherein not more than one of the constituents is 0wt. %.
 3. Lead substitute material according to claim 2, characterisedin that the lead substitute material comprises from 12 to 22 wt. %matrix material, from 0 to 39 wt. % Sn or Sn compounds, from 16 to 60wt. % W or W compounds and from 16 to 60 wt. % Bi or Bi compounds. 4.Lead substitute material according to claim 1, characterised in that thelead substitute material comprises from 12 to 22 wt. % matrix material,from 40 to 60 wt. % Sn or Sn compounds, from 7 to 15 wt. % W or Wcompounds and from 7 to 15 wt. % Bi or Bi compounds.
 5. Lead substitutematerial according to claim 1, characterised in that the lead substitutematerial additionally comprises up to 40 wt. % of one or more of thefollowing elements: Er, Ho, Dy, Th, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I,Pr and/or their compounds and/or CsI.
 6. Lead substitute materialaccording to claim 5, characterised in that the lead substitute materialadditionally comprises up to 20 wt. % of one or more of the followingelements: Er, Ho, Dy, Th, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I, Pr and/ortheir compounds and/or CsI.
 7. Lead substitute material according toclaim 6, characterised in that the lead substitute material additionallycomprises up to 8 wt. % of one or more of the following elements: Er,Ho, Dy, Th, Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I, Pr and/or their compoundsand/or CsI.
 8. Lead substitute material according to claim 1,characterised in that the lead substitute material additionallycomprises up to 40 wt. % of one or more of the following elements: Ta,Hf, Lu, Yb, Tm, Th, U and/or their compounds.
 9. Lead substitutematerial according to claim 8, characterised in that the lead substitutematerial additionally comprises up to 20 wt. % of one or more of thefollowing elements: Ta, Hf, Lu, Yb, Tm, Th, U and/or their compounds.10. Lead substitute material according to claim 9, characterised in thatthe lead substitute material additionally comprises up to 8 wt. % of oneor more of the following elements: Ta, Hf, Lu, Yb, Tm, Th, U and/ortheir compounds.
 11. Lead substitute material for radiation protectionpurposes in the energy range of an X-ray tube having a voltage of from60 to 90 kV according to claim 5, characterised in that for nominaloverall lead equivalents of from 0.25 to 0.6 mm the lead substitutematerial comprises from 12 to 22 wt. % matrix material, from 49 to 65wt. % Sn or Sn compounds, from 0 to 20 wt. % W or W compounds, from 0 to20 wt. % Bi or Bi compounds and from 2 to 35 wt. % of one or more of theelements Gd, Eu, Sm, La, Ce, Nd, Cs, Ba, I, Pr and/or their compoundsand/or CsI.
 12. Lead substitute material according to claim 11,characterised in that the lead substitute material additionallycomprises from 2 to 25 wt. % I, Cs, Ba, La, Ce, Pr and/or Nd and/ortheir compounds and/or CsI.
 13. Lead substitute material for radiationprotection purposes in the energy range of an X-ray tube having avoltage of from 100 to 140 kV according to claim 5, characterised inthat for nominal overall lead equivalents of from 0.25 to 0.6 mm thelead substitute material comprises from 12 to 22 wt. % matrix material,from 40 to 73 wt. % Bi and/or W or their compounds and from 5 to 38 wt.% of one or more of the following elements: Gd, Eu, Er, Hf and/or theircompounds.
 14. Lead substitute material according to claim 1,characterised in that it comprises a structure of protective layers ofdifferent compositions.
 15. Lead substitute material according to claim14, characterised in that it comprises a structure of at least twoprotective layers of different compositions which are separate or joinedtogether, wherein the protective layer(s) more remote from the bodycomprise(s) predominantly the elements having a lower atomic number, ortheir compounds, and the protective layer(s) close to the bodycomprise(s) predominantly the elements having a higher atomic number, ortheir compounds.
 16. Lead substitute material according to claim 14,characterised in that it comprises a structure of at least twoprotective layers of different compositions which are separate or joinedtogether, wherein at least in one layer at least 50% of the total weightconsists of only one element from the group Sn, W and Bi or theircompounds.
 17. Lead substitute material according to claim 14,characterised in that it comprises a structure of at least twoprotective layers of different compositions which are separate or joinedtogether, wherein at least in one layer at least 50% of the total weightconsists only of at least 40 wt. % Sn or its compounds and at least 10wt. % I, Cs, Ba, La, Ce, Pr and/or Nd and/or their compounds and/or CsI.18. Lead substitute material according to claim 14, characterised inthat it comprises a structure of at least two protective layers ofdifferent compositions which are separate or joined together, whereinthe protective layer(s) more remote from the body comprise(s)predominantly the elements or their compounds having a higher X-rayfluorescent yield, and the protective layer(s) close to the bodycomprise(s) the elements or their compounds having a lower X-rayfluorescent yield.
 19. Lead substitute material according to claim 14,characterised in that it comprises a structure of at least threeprotective layers of different cmpositions which are separate or joinedtogether, wherein the protective layer(s) more remote from the body andthe protective layer(s) close to the body comprise predominantly theelements having a higher atomic number or their compounds, and there isarranged in the middle at least one protective layer comprisingpredominantly elements having a lower atomic number.
 20. Lead substitutematerial according to claim 14, characterised in that a weaklyradioactive layer is embedded between two non-radioactive protectivelayers which are separate from or joined to the radioactive layer. 21.Lead substitute material according to claim 1, characterised in that themetals or metal compounds are granular and their particle sizes exhibita 50^(th) percentile according to the following formula$D_{50} = {\frac{d \cdot p}{10}{mm}}$ wherein D₅₀ represents the 50^(th)percentile of the particular size distribution, d represents the layerthickness in mm and p represents the proportion by weight of theparticle material component in the total weight, and the 90^(th)percentile of the particle size distribution D₉₀≦2·D₅₀.
 22. Radiationprotection apron of lead substitute material according to claim 1.